Ok, thanks for support team, I could solve that. If anyone need do it, just follow these steps:

1. add an A record “www” with your vps’s ip as value
2. go to /var/www/ghost and run ghost config url https://www.mydomain.com
3. then, generate a ssl setup running ghost setup nginx ssl
4. set again your main domain ghost config url https://mydomain.com
5. edit the nginx config files for your second domain to redirect to your main domain: edit both: /system/files/my-second-domain.com.conf and /system/files/my-second-domain.com-ssl.conf , replacing the content of the first location block (leave the .well-known location block - this is used for renewing certificates) with:

return 301 https://my-canonical-domain.com$request_uri;

1. to finish run sudo nginx -t and then sudo nginx -s reload

^-^